A plasma processing apparatus generates a plasma in a chamber which can be used to treat a workpiece supported by a platen in a process chamber. In some embodiments, the chamber in which the plasma is generated is the process chamber. Such plasma processing apparatus may include, but not be limited to, doping systems, etching systems, and deposition systems. In some plasma processing apparatus, such as ion assisted deposition, ions from the plasma are extracted and then steered towards a workpiece. In a plasma doping apparatus, ions may be accelerated to a desired energy so as to create a certain dopant depth profile in the physical structure of the workpiece, such as a semiconductor substrate.
In some implanters, the plasma may be generated in one chamber, which ions are extracted from, and the workpiece is treated in a different process chamber. One example of such a configuration may be a beam line ion implanter where the ion source utilizes an inductively coupled plasma (ICP) source.
Turning to FIG. 1, a block diagram of one exemplary plasma processing apparatus 100, which uses inductive coupling, is illustrated. The plasma doping apparatus 100 includes a plasma chamber 101 used to generate ions and a processing chamber 104 used to implant semiconductor wafers. A dielectric window 102 (usually made of quartz, alumina or sapphire) is used to couple the electrical power from an RF generator 151 to the working gas. At the opposite side of the plasma chamber 101 an extraction plate 103 having an extraction slit 105 or an array of extraction slits of different geometries is used for extraction of ions. The working gas is introduced in the plasma chamber 101 through gas inlets 106 symmetrically distributed with respect to the extraction slit 105 and in such geometry to ensure a uniform gas flow in the plasma chamber 101 cross-section.
Plasma is generated inside the plasma chamber 101 by coupling the RF power from the RF generator 151 to a pancake type or planar antenna 152. The variable plasma impedance is matched with the 50Ω generator impedance by a matching network 153.
Plasma uniformity may be improved by a magnetic multicusp configuration composed of magnets 107, which may be permanent magnets. The magnetic field strength of the magnets 107 is enhanced by the steel yokes 108, which act to close the field lines outside plasma chamber 101. The magnets 107 are arranged in an alternating pattern so that the magnetization direction points alternatively inward and outward of plasma chamber 101. In this way the multicusp field lines geometry prevent charged particles for being lost at the walls, thus increasing the plasma density and uniformity. To reduce the level of impurities in the plasma that might come from the sputtering of the walls, thin liners 109 made of SiC, quartz, or Si sprayed Al can be used.
Depending on the desired dopant species (typically P for n-type doping and B for p-type doping, but other species such as As, Ge, Ga, In, etc. may also be used) different feedstock gases containing the dopant atoms can be fed into the plasma chamber 101 at variable flow rates by a gas manifold 111. The gas manifold is comprised of gas containers 112, valves 113, and mass flow controllers 114. The vacuum pumping is performed through the extraction slit 105 by a turbomolecular pump 115 backed by rotary pump 116. In other embodiments where an independent control of the flow rate and pressure in the plasma chamber 101 is desired, a separate pumping line for the plasma chamber 101 may be used. Because the photoresist that is present on the wafer releases large amount of hydrogen during the implantation process, in addition to turbomolecular pump 115 and rotary pump 116, a cryo pump 117 may be used to pump the processing chamber 104 due to its high efficiency at pumping out hydrogen. The pressure in the plasma chamber 101 and the processing chamber 104 is monitored by a Baratron gauge 118 and a Bayard-Alpert gauge 119, respectively.
To extract positive ions, the plasma chamber 101 is maintained at a positive electric potential by an extraction power supply 121 while the processing chamber 104 may be at ground potential. A high voltage bushing 122 ensures electrical insulation between the plasma chamber 101 and processing chamber 104.
The ion beam 130 is extracted from the plasma source by a triode (a three electrode electrostatic lens) composed of the face plate 103, suppression electrode 131 and ground electrode 132 electrically insulated one from each other by ceramic high voltage insulators 133. In other embodiments, a tetrode (four electrode lens) or a pentode (five electrode lens) may be used for ion beam extraction. Because the plasma chamber 101 is at positive potential, positive ions are pulled out from the chamber 101 by the ground electrode 132. Most of the extracted ions will pass through the slit 134 in the ground electrode 132 but some will strike the ground electrode 132. When such an event occurs, secondary electrons are generated. To prevent secondary electrons from streaming back toward the source, the suppression electrode 131 which is interposed between the extraction electrode 103 and the ground electrode 132 is polarized at negative potential by a suppression power supply 135. The connection of the suppression power supply 135 with suppression electrode 132 is achieved through a high voltage feedthrough 136.
The extracted ion beam 130 is steered toward the wafer 143 which is disposed on a platen 142, which may be grounded. The platen 142 may be adapted able to move back and forth to expose the entire wafer surface to the ion beam 130.
Since it is on the ion source side, the whole RF system floats at the extraction potential. The antenna 152 has one leg connected to the matching network 153 output and the other leg at the elevated ground. In some embodiments, a high voltage capacitor (not shown) is inserted in the ground leg to make an even voltage distribution over the antenna 152 length. Once the RF power is applied, an RF current start to flow through antenna 152. The RF current generates a time varying magnetic field, which, according to Maxwell's 3rd electrodynamics law induces an electric field in the proximity of the antenna 152. Because of long mean free path due to the lower pressure in the plasma chamber 101, the electric field is able to accelerate free electrons to such an energy that a collision with a gas particle (atom, molecule) will result in an ionization process. Most of the RF power for deposition and, implicitly the ionization processes, occur in the vicinity of the dielectric window 102 in a skin layer of few Debye length.
Continuous operation may lead to wafer charge build-up, followed by catastrophic damage to the features on the wafer 143. Therefore, in some embodiments, the ion beam 130 is pulsed. A pulse modulator 161 can drive synchronously both the extraction power supply 121 and the suppression power supply 135, thereby allowing changing of the pulse frequency and duty cycle.
In some embodiments, the planar antenna 152, might have a spiral-like shape as shown in FIG. 2A or serpentine like shape as shown in FIG. 2B. In all cases, the antenna 152 is made of electrically conductive material, such as aluminum, copper or silver plated copper, preferably in a tubular shape to allow water cooling. In other embodiments, the whole antenna 152 is immersed in a dielectric resin, thus allowing direct contact between the antenna 152 and the dielectric window 102.
Inductively coupled plasma source operation is based on energy transfer from the RF power generator 151 to the plasma electrons via an antenna 152. However, in the initial stage of the discharge, the RF power is coupled capacitively (E mode operation), therefore the antenna 152 acts as a plate of a capacitor. In such cases, the electrons gain energy from the electric field in the direction perpendicular to the plane containing the antenna 152 either through ohmic heating or stochastic heating. Larger plate area creates better capacitive coupling and implicitly easier gas breakdown. Once the plasma is ignited, the RF coupling evolves toward the inductively coupled mode (H mode operation), but some capacitive coupling still remains. Because there are inherent losses in the antenna 152 and the matching circuit 153, the coupling efficiency (η) of the antenna 152 is usually in the range between 0.6 and 0.8. Higher coupling efficiency means better electron heating, a larger number of ionization events and, in fact, higher plasma density and implicitly higher extracted ion beam current. Assuming perfect RF matching (i.e. zero reflected power) and negligible losses in the matching capacitors, antenna efficiency is given by the ratio of the amount of power transferred to the plasma (Pp) to the total amount of power delivered by the generator (PG). It can be related to the power loss in the antenna (Ploss)η=Pp/PG≅|−Ploss/PG  (1)
From Eq. (1) it can be seen that one way to increase the power coupling efficiency is by decreasing the amount of power losses (Ploss).
As shown in FIG. 3, H mode operation is characterized by high plasma density (>5×1010−1×1012 cm−3), low plasma potential, and low electron temperature (<3 eV) whereas E mode operation is characterized by low plasma density (<1×1010 cm−3), high plasma potential and high electron temperature. The triangles in FIG. 3 represent the plasma potential (which scales proportionally with electron temperature). The value of the plasma potential can be read on the right scale. The circles in FIG. 3 represent the plasma density, the values of which can be read on the left scale. For certain applications it is desired that some processes to be run in H mode while others in E mode.
Therefore, it would be beneficial if there were a system that will allow in situ switching from H mode to E mode and vice versa. Also, when running in H mode, it would be beneficial to have a system and a method for increasing the coupling efficiency and therefore, boosting the plasma density. Such a system may also advantageously reduce the cooling requirements for the antenna.